This progression builds on the last few lessons. It uses ascending passing diminished chords (which function as dominant 7ths) to approach the II-7 and III-7 chord, and then proceed down a III-7 VI7 II-7 V7 turnaround. This sort of substitution is common in many jazz tunes. You don’t need a lot of different chord forms to play it. Notice how many uses our Diminished 7th chord form can be put to.